Regulatory documents on the subject of oil-and-gas well operation safety require to conduct all wellbore operations through tubing assembly. The most common tubings are 60-89 mm in diameter, therefore downhole devices used in a bottom-hole formation zone must have maximum diameters of 44-52 mm.
The known method [1] is based on the excitation of a downhole ultrasonic emitter by an electric signal within a process frequency range, the energy conversion of an electric signal into acoustic vibrations energy. A near productive zone of a well is affected by acoustic vibrations of the sum of electric signals of frequency series within a process frequency range. A distant productive zone is affected by low-frequency acoustic vibrations of combinative residual frequencies within a process range. The device comprises series-connected: control device comprised of a multi-channel master frequency oscillator and a multi-channel phase-pulse modulator; generating device including a number of power amplifiers; coordinator; cable; downhole ultrasonic emitter; power rectifier.
While applying this method for a treatment of a productive well zone in a perforated interval, a treatment increment is 1-2 meters, a downhole ultrasonic emitter with the active length of 0.5-1.5 meters and the acoustic power of 0.5-5 kW, wherein at each increment a downhole ultrasonic emitter at first is excited by a tonal frequency-modulated signal during 0.1-1 hour and then is excited by an electric signal in the form of the sum of electric signals of frequency series within a process frequency range, including frequency-modulated ones, during 1-4 hours. Frequencies within a process range are set in the range of 10-60 kHz, taking into account geological characteristics of a near productive well zone, so that combinative difference frequencies fall in the range of 20-4000 Hz taking into account geology of a distant productive well zone.
Noting the appropriateness of the aim—to ensure efficiency of an impact on an oil formation for improving a production rate of a well due to the influence of high-frequency and low-frequency vibrations, it should be noted that the authors do not fully understand the physics and the objectives of an impact on a bottom-hole formation zone by various frequencies. An impact of high-frequency vibrations (16-30 kHz) provides a clearance of perforation tunnels and a bottom-hole zone of colmatant and other types of clogging. Away back in the 1970s, it was experimentally proved that the most effective frequencies for this task are the frequencies close to 20 kHz [2, 3]. Low-frequency vibrations [3, 4] are intended precisely to enhance oil recovery of a formation by the initiation of filtration flows in a reservoir in general. The greatest impact is reached when properties of low-frequency vibrations are close to properties of resonating vibrations of an oil formation [4].
The detailed description of the control device producing specified output signals is given but there is almost no operation description of the downhole ultrasonic emitter. Meanwhile, the proposed equipment must be considered as a unified system providing the furtherance of the desired objective. Signals formed in the output of the control device are different from acoustic emitter output signals in means of frequency and power. In the downhole emitter there is a piezoceramic transducer converting electric signals into mechanical signals. For such emitters the piezoceramics with high Q-factor is used in order to get the maximal vibrations amplitude (intensity of the radiation) at a resonating frequency [5]. However, with higher Q-factor of piezoceramics the range of resonating frequency is narrower (FIG. 1). In the invention it is also proposed to set a wide process range of frequencies 10-60 kHz. It means that the emitter would have the maximal power level (intensity of the radiation) only on one specified frequency, but on other frequencies the efficiency would be close to zero. Let us explain it by an example. The intensity of radiation drops sharply in passing through the interface of two mediums. The known dependence is [6]:I2=I1(4c1p1/c2p2)/{c1p1/c2p2+1}2, where
I1, I2—are the intensities of radiation of the first and the second mediums,
c1, c2—sound velocity in medium 1 and medium 2, respectively,
p1, p2—density of medium 1 and medium 2, respectively.
After we substitute the values of the two mediums—oil and steel (well casing), we can see that the intensity of radiation just by so doing would be decreased more than 3 times.
It is also stated in the description that the emitter comprises several piezoceramic packages. Meanwhile, the control device does not provide the use of an automatic frequency maintaining system to provide the simultaneous operation in the mode maximally close to resonating one.
The above-mentioned method does not provide a pressure drawdown in a perforation zone and elimination of clogging materials from a well and bottom-hole formation zone. However, acoustic technologies experience of numerous oilfield service companies [2, 3] and the experience of the authors of the invention shows that deployment of this technology significantly increases (2-3 times) the duration of the acoustic treatment effect because broken physico-chemical bonds of clogging materials recover again and clog a bottom-hole zone and perforation tunnels.
There is also a method [7] of recovery and maintaining productivity of a well including an acoustic impact on a well and formation implemented cyclically, in the presence of a pressure gradient between well and formation, with the beginning of a cycle at maximum pressure differential between well and formation in the time of declining of well production rate or well intake capacity and the end of a cycle while achieving the stabilization in the increase of well production rate/intake capacity or stopping the flow between well and formation.
The pressure gradient is produced by a high-productivity pump installed at the deepest possible depth and operated in the mode of producing alternating pressure drawdowns, once maximally pumping out all the fluid from a well and producing a maximal pressure drawdown, once halting for fluid accumulation, wherein the formation is loaded with considerable and alternating pressure drawdowns with simultaneous acoustic impact or in the presence of the flowing effect the natural pressure gradient between a well and formation is used.
An impact is applied by an acoustic emitter plunged into a well simultaneously with downhole equipment during well development or workover process before running a well, an acoustic emitter is installed in a perforated formation zone or a selected medium with a possibility of influencing on a productive (perforated) formation zone, by means of, for example, choosing the appropriate length of an emitter or number of series-connected emitters.
A drawback of this method is that a downhole emitter is installed at one fixed point in a well. In case of a large thickness of a formation or a large number of mediums only the zone near the emitter would be treated and other zones would not be. A pressure drawdown and continuous fluid removal from a well enable to remove degradation products of clogging materials in time and increase the lifetime of the effect. However, the use of a high-productivity pump operated in the mode of producing alternating pressure drawdowns in a bottom-hole zone increase the cost of oil production and risks of expensive downhole equipment failure. In the description of the method it is stated that there is some pulse mode of the emitter but there is no description of how it is carried out. There is also no description of the operation modes (frequency, intensity, time, etc.), which makes it impossible to compare the inventions fully.
There is also a known method similar to the invention in operational principles and taken by the claimants as a prototype [8]. The method includes the installation of a downhole device in a well at the operational depth connected to a surface industrial frequency power supply and comprising an ultrasonic transducer providing elastic vibrations of high frequency, an excitation of elastic vibrations of various frequencies and subsequent repeated impact by elastic vibrations on an oil formation. This impact is carried out by high and/or low frequency vibrations.
To generate elastic vibrations of high and low frequency there are used two independent sources of vibration, one of which is at least one emitting ultrasonic, mostly magnetostrictive, transducer and the second one is based on an electropulse device, which provides generation of low frequency elastic vibrations, connected to a surface industrial frequency power supply and comprises electrically interconnected: a charging unit, a unit of storage capacitors, a discharge unit provided with electrodes, two switching units, one of which configures separate storage capacitors into one single unit and the second one provides switching of storage capacitors to other type of connection. Meanwhile, the impact of high-frequency elastic vibrations is carried out within a low-frequency ultrasound range, mostly at the frequency of 18-44 kHz, and the impact is conducted in the continuous and/or the pulse mode within the intensity of 1-5 W/cm2. The impact of low-frequency elastic vibrations is carried out with the pulse discharge frequency of 0.2-0.01 Hz and the impact is conducted with the energy of single discharge pulse which is 100-800 J.
It is proposed to use a magnetostrictive device as an ultrasonic emitter. Such a device has only one emitting point—in the center of a waveguide, whence the acoustic waves approximate in shape to the ellipsoid shape are emitted into space. The primary emission goes in the radial direction. Thus, in a fixed state it only treats a narrow strip of bottom-hole zone. To provide an effective bottom-hole formation zone cleaning, especially perforation zone of a well, it must be moved along a well in very short increments of 0.2-0.3 m, which increase the time of treatment significantly.
It is proposed to use an electrohydraulic device with the diameter of 106 mm as a low frequency emitter. Such a device can only be used only in a casing pipe, therefore, to provide safety, the authors propose to use the method in which both devices are attached to the tubing and are lifted down to the treatment zone. Thus, such a configuration can only treat a zone with the width of 2-3 meters, and the other formation zones will remain untreated.
In the considered method the use of pressure drawdown is stipulated (by means of a pump or a swab), as well as the fluid removal with broken clogging materials from the bottom-hole zone. However, the aforementioned drawback will not bring any significant effect, especially in wells with centrifugal pumps.